In an image forming apparatus of an electro-photographic method, a toner image is formed on an image bearing member such as a rotating photoreceptor drum or a moving photoreceptor belt, and the formed toner image is transferred onto a transfer body such as a transfer drum or transfer belt, then, further transferred onto a recording medium from the transfer body, and formation of an image is thus performed.
Particularly, in an image forming apparatus which forms a full color image, a method is employed in which a toner image of each color is formed on each of a plurality of photoreceptors, and the toner image of each color, having been formed, is superposed on a transfer body, and formation of a full color image is thus performed.
In those cases, image transfer needs to be carried out while the photoreceptor and the transfer body rotate or move at the same surface speed. Differences between the surface speeds of the photoreceptor and the transfer body may cause a visible image drift or color drift.
Also, in cases in which the photoreceptor and the transfer body contact each other under the condition that the surface speeds of the photoreceptor and the transfer body are not the same, because the friction coefficient between the photoreceptor and the transfer body varies significantly depending on the existence or non-existence of toner between the photoreceptor and the transfer body, load fluctuation on both sides of the photoreceptor and the transfer body increases, resulting in a possible driving error.
Because of this, it is preferable to rotate or move the photoreceptor and the transfer body at the same surface speed. However, due to the influence of the dimensional variations occurring in the diameter of photoreceptor drum, the diameter of transfer body driving roller, the thickness of transfer belt, and the like, an image drift or color drift may occur even if the motor and drive shaft are controlled so as to rotate or move the photoreceptor and the transfer body at a same predetermined driving speed, because a relative difference in surface speed between the surfaces of the photoreceptor and the surface of the transfer body actually occurs.
In such a case, a technique is disclosed by Unexamined Japanese Patent Application Publication No. 1985-42771 (hereinafter referred to as Patent Document 1) as follow.
According to Patent Document 1, it is possible to maintain the moving speed at the surfaces of the photoreceptor or the transfer body to be constant, without the effect of errors in diameters of the photoreceptor and the transfer body driving roller, by: a) providing detected sections for speed detection placed on the surface of the photoreceptor or the surface of the transfer body at a predetermined interval, b) detecting the detected sections by a sensor, and c) carrying out feedback to control the rotation speed of a motor, which drives the photoreceptor or the transfer body, based on the results detected by the sensor.
However, in order to carry out such control, it is necessary to provide detected sections exclusively on a photoreceptor and a transfer body. Further, it is necessary to provide a sensor near the detected sections. As a result, the manufacturing cost of the apparatus is likely to increase.